1. Field of the Invention
This invention relates to an anticorrosive insulator such as an anticorrosive suspension insulator for use in insulator strings to be supported by arms of transmission line towers.
2. Related Art Statement
Referring to FIG. 6, a typical suspension insulator uses an insulator body 1 having a shed 1a extending radially from a central core 1c. A metal cap 3 is firmly secured to the top of the core 1c by cement 2. A metal pin 4 is inserted to the inside of the core 1c and secured thereto by cement 2a. In the conventional suspension insulator, the spacing or the gap g (FIG. 7) between the lower end of the metal cap 3 and the upper surface of the shed 1a has been less than 2 mm. As shown in FIG. 7, the bottom surface of the cement 2 between the metal cap 3 and the core 1c is generally finished flush with the lower end of the metal cap 3.
If the suspension insulator is used in a DC (direct current) power transmission line in such a manner that the polarity in the metal cap 3 is positive and the polarity in the metal pin 4 is negative, a surface leakage current flows from the metal cap 3 to the metal pin 4 along the surface of the shed 1a, and such leakage current causes electrochemical corrosion (to be referred to as electric corrosion hereinafter) at the lower end of the metal cap 3. Since the spacing between the lower end of the metal cap 3 and the upper surface of the shed 1a is less than 2 mm in conventional suspension insulators, and since the bottom surface of the cement 2 is flush with the lower end of the metal cap 3, corrosion products due to the above electric corrosion collect in the very small space surrounded by the metal cap 3, the bottom surface of the cement 2, and the shed 1a.
As the amount of the corrosion products deposited on the sturdy metal cap 3 increases, the comparatively hard corrosion products tend to generate a local pressure on the surface of the shed 1a. When the stress concentration in the shed 1a due to such local pressure exceeds a certain limit, cracks C are produced in the shed 1a of the insulator body 1, as shown in FIG. 8. Susceptibility to such cracks C due to electric corrosion is a weak point of the conventional insulators, because the presence of the cracks C weakens the insulator and invites breakdown of the insulator upon exposure to electric and mechanical stresses.